Bearings in operation, due to various complicated reasons, such as unreasonable structure, poor material quality, low performance, defects on the working surface, impact, vibration, improper installation and poor lubrication, can cause early failure of the bearing. However, in some cases, some bearings are not put into use (running) at all, but the original intact appearance of the user during the installation process is damaged, and the use is lost, resulting in premature bearing failure. What is revealed in this paper is a practical example of this form of failure.
First, the failure background
When a power machine factory assembled the S195 diesel engine gearbox, there was a phenomenon that the inner ring of the 6205 deep groove ball bearing was intermittently or continuously cracked (block), occupying a certain proportion in quantity (four sets per gear box). According to the site, the total supply of the batch of bearings is 3,000 sets, and 1000 sets have been installed. The remaining 2,000 sets are not ready for installation due to the user’s “heart and soul”.
It has been verified that the dimensions of the gearbox shaft meet the requirements of the drawings, and the bearing installation method is correct, which is completed by a fixed skilled worker. The user has extracted 40 sets of the inner and outer rings of the bearing and measured the hardness of the inner and outer rings of the bearing. The hardness of the DZ-2000 is detected in the range of 61 to 65 HRC, which meets the JB1255 standard. No cracks are found by visual inspection. In view of this, in order to plead for the cause of cracking of the bearing ring, the damaged bearing on the site was returned, and conventional physical and chemical tests were carried out.
These ferrules are heated and quenched by a box type resistance furnace at 840 to 860 ° C, and tempered at 160 ° C × 2 ~ h low temperature oil.
Second, test methods and results
Macroscopic observation
Figure 1 is a photo of the bearing that was damaged during installation.
After careful observation of the real thing, we can know:
(1) No damage marks left by the installation of the machine on all the damaged bearings, and the steel balls, cages and other parts are intact.
(2) The cracking of the inner ring starts at the side of the center of the raceway and occupies about 1/2 or 1/4 of the diameter of the end face.
(3) There is a secondary longitudinal straight crack extending into the inner diameter surface at the fracture, and the crack is shaped like a knife and penetrates the wall surface.
(4) The shape of the entire crack is uneven, but the surface of the fracture is smooth and flat. There are no signs of plastic deformation and only a few tear marks.
(5) The fracture is gray and fine grained porcelain brittle fracture.
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